Process for the decarbonization of steel

ABSTRACT

A PROCESS FOR THE DECARBONIZATION OF ELECTRICAL SHEET STEEL WHICH COMPRISES COMBINING THE DECARBONIZATION WITH THE DEVELOPMENT OF MAGNETIC CHARACTERISTICS OF THE STEELINTO ONE SINGLE STEP BY CONTACTING A STEEL STRIP OR SHEET AT 1,000*C. TO 1,300*C. WITH A MIXTURE OF WATER VAPOR AND HYDROGEN WHEREIN THE RATIO OF THE PARTIAL PRESSURE OF WATER VAAPOR TO THE PARTIAL PRESSURE OF HYDROGEN IS CONTROLLED BETWEEN 2X10**-4 TO 3X10**-3, AT A TEMPERATURE OF 1,00*C. AND 2X10**-4 TO 10**-1 AT A TEMPERATURE OF 1,300*C.

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FROCESS FOR THE DECARBONIZATION OF STEEL Filed Feb. 22, 1972 4 Sheets-Shoat 2 T c) H 0 DOT 1,050 6,2X l0 DOT 5 1,300 5,2x l0" Fig. 2

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7) H/PFTJ E 600 C, P /P 340" 2) J56. 575p 1100 "c, P P 4-10" 1100 "c, PHZO/PHZ =9-10- Z 4 6 0 l0 AMI/44 /A/@ 7/04.? [/W/AQ United States Patent Office 3,823,042 Patented July 9, 1974 3,823,042 PROCESS FOR THE DECARBONIZATION OF STEEL Fritz Bolling, Homberg, Armin Mayer and Mavendra Parikh, Dinslaken, and Friedrich Toussaint, Wulfrath- Rutzkausen, Germany, assignors to Stahlwerke Bochum Aktiengesellschaft, Bochum, Germany Filed Feb. 22, 1972, Ser. No. 227,921 Claims priority, application Germany, Feb. 20, 1971, P 21 08 242.8 Int. Cl. H01f 1/04 US. Cl. 148-112 2 Claims ABSTRACT OF THE DISCLOSURE A process for the decarbonization of electrical sheet steel which comprises combining the decarbonization with the development of magnetic characteristics of the steel into one single step by contacting a steel strip or sheet at 1,000 C. to 1,300 C. with a mixture of water vapor and hydrogen wherein the ratio of the partial pressure of water vapor to the partial pressure of hydrogen is controlled between 2 1O to 3x10 at a temperature of 1,000 C. and 2X10- to at a temperature of 1,300 C.

BACKGROUND OF THE INVENTION Field of the Invention This invention is directed to the decarbonization of electrical strip or cold-rolled sheet steel. More particularly, this invention is directed to the simultaneous decarbonization and the development of magnetic properties in electrical strip or sheet steel. This invention is also directed to such a decarbonization and development of magnetic properties in the development of a specific steel having a given silicon and carbon content.

DISCUSSION OF THE PRIOR ART The present-day, commonly practiced methods of finally annealing high-alloy, cold-rolled electrical strip or sheet steel are mostly characterized by a two-step procedure. The material is decarbonized at a temperature of about 750 to 900 C., under a relatively moist, hydrogen-rich atmosphere, and highly annealed at a temperature of 1,000 to 1,200 C., under a relatively dry furnace atmosphere to improve its magnetic characteristics. Typical examples of such methods are described in US. Pat. No. 2,599,340 and in German Auslegeschrift 1,182,276.

By a moist atmosphere is meant an atmosphere in which the ratio between the water vapor content and the hydrogen content amounts to more than 0.02, and as a rule to about 0.1-0.4. Furthemore, the atmosphere can contain nitrogen or other components, for example, but the composition is usually such that the atmosphere is a reducing atmosphere for iron. On the other hand, when one speaks of a dry atmosphere or of dry hydrogen, the water vapor content is substantially less than about 1 to 2% of the hydrogen content. Further information in this regard is contained in US Pat. No. 2,287,467 and German Auslegeschrift. 1,182,276.

German Auslegeschrift 1,259,923 also discloses a method in which a decarbonization reaction is performed in a higher temperature range of from 1,065 to 1,205" C. In this range, cold-rolled sheets are partially decarbonized in a moist, preferably oxidizing atmosphere, then pickled, and finally decarbonized to a final carbon content at 760 to 927 C. in an atmosphere that is reducing for iron. A final heat treatment in the high temperature range to accomplish complete decarbonization is considered undesirable because in that case a silicon dioxide coating is left on the sheet surface. Such coating is difficult to pickle off and very greatly impairs the punchability of the material.

According to German Pat. No. 861,702, it is known to perform the decarbonization and at the same time develop the magnetic characteristics with a single heat treatment in a temperature range of 1,040 to l,175 C. However, no guidance for moisture content of the atmosphere is given in the patent. This process is applicable only to a sheet which, in contrast to the processes mentioned hitherto, is covered with scale on its surface. The scale plays an important role in the decarbonization reaction in German Pat. No. 861,702.

In the process of Austrian Pat. No. 246,203 only one heat treatment is performed, which heat treatment is at a temperature of 800 to 1,200 C. This treatment is intended to produce both the deear bonizaton and the development of the magnetic properties. An atmosphere of moist hydrogen is recommended. In this process, the surface of the sheet is coated with a salt which probably acts as a catalyst in the decarbonizing reaction. It is unknown whether ths process has proven practical.

Additionally, a process is mentioned in German Auslegeschrift 1,005,098 in which the decarbonization is performed in chemically pure hydrogen at temperatures of 1,200 to 1,400 C. The decarbonization is said to follow the reaction 2H +C- CH A high velocity of flow of hydrogen is considered necessary in the process. This process, however, has apparently never managed to come into practical application anywhere. Experiments have shown that the amount of methane that developed is extremely small. Hydrogen is generally considered to be unsuitable for economical decarbonization because the reaction is an extremely slow one (see US. Pat. No. 2,287,467).

OBJECT OF THE INVENTION The object of the invention is to provide a process of the kind initially described, by which an improved surface quality and improved magnetic properties can be achieved in the electrical strip or sheet steel in a simple and economical manner.

SUMMARY OF THE INVENTION This invention contemplates the simultaneous decarbonization and development of magnetic properties in electrical strip or sheet steel by a process which comprises heat treating said steel at a temperature of at least 1,000- C. in an atmosphere in which the partial pressures of water vapor and hydrogen are especially controlled.

In accordance with this invention, a cold-rolled electrical strip or sheet steel having electrical properties, is subjected to a heat treatment of 1,000 C. in the presence of a mixture of hydrogen and water vapor such that the ratio of the partial pressure of the water vapor and hydrogen is in the range of 2 l0 to 3 l0- It should be understood that the present invention is carried out with a mixture of water vapor and hydrogen. The ratio of these partial pressures varies inversely with the temperature employed.

For example, at a temperature in the range of 1,300 C. is employed, it is desirable that the ratio of the partial pressure of water vapor to the partial pressure of hydrogen is between '2X10- to 10- An especially desirable va is 5X10- to 2x10 On the other hand, if the heat treatment is performed at the lower end of the preferred range, i.e., at about 1,100 C., a suitable ratio for these partial pressures is 5X10" to 2X10.

It has been found in accordance with the present invention that not only can the steel be effectively decarbonized without the creation of a scale on the surface, but the development of magnetic properties can be accomplished. Thus, in accordance with the present invention, there has been provided a steel strip or sheet having a low carbon content with a comparatively low coercive field strength in oersteds.

In an especially desirable embodiment, the present in vention contemplates the decarbonization and development of magnetic properties in electrical strip or sheet steel, which steel has a silicon content of between 2 and 6 weight percent and preferably between 2 and 4 weight percent, of which no more than 2% and preferably no more than 0.5% of said silicon can be replaced by aluminium and no more than 0.008% by carbon, the balance being substantially iron. This steel preferably has a carbon content of less than 0.005 weight percent. The steel can contain the usual, generally unavoidable impurities, such as manganese, sulfur and phosphorus. The strip or sheet is one which is a scale-free sheet and cold-rolled to a final thickness.

DISCUSSION OF PREFERRED EMBODIMENTS In accordance with the invention, the final annealing consists of a single final heat treatment at a temperature above 1,000 C. which serves both for decarbonization and for the development of the magnetic properties. In this heat treatment the atmospheric moisture content lies within a quite specific range depending on the temperature. At a heat treating temperature of 1,000 C., the partial pressures of water vapor and hydrogen are, in accordance with the invention, in a ratio of 2X 10* to 3 X 10*, and at a temperature of 1,300 C., they are in a ratio of 2X 10* to 10- At temperatures within this temperature range, the partial pressures are in a correspondingly interpolated ratio to one another. The upper limit of the heat treating temperature is determined solely by the technical possibilities, especially by the mechanical strength of the sheet which diminishes with increasing temperature. The heat treatment atmosphere is only very slightly in motion, this movement being brought about only by the fact that an inflow of hydrogen is required to sustain the regeneration and to compensate leakage losses.

The heat treatment is performed to special advantage at a temperature above l,l C. in an atmosphere in which the partial pressures of water vapor and hydrogen at a temperature of 1,100 C. are in a ratio of 5 10=- to 2 10- and at a temperature of l,300 C. in a ratio of 5X10' to 2X and at temperatures within this temperature range, they are in a correspondingly interpolated ratio.

The heat treatment in accordance with the invention is performed preferably in a continuous manner. Batch heat treatment of coils or packs is also possible, however, although free access of the atmosphere to the surface must, of course, be assured. It is furthermore recommended in accordance with the invention that the heat treatment be performed in an atmosphere consisting exclusively of hydrogen and water vapor. If other gases, such as nitrogen or argon, are present, the water vapor content is determined solely on the basis of the hydrogen content in accordance with the ratios specified above.

EXAMPLE A strip of cold-rolled, roll hardened sheet 0.5 mm. thick was available for the test. The melt analysis was as follows: 0.015% carbon, 0.21% manganese, 0.025% phosphorus, 0.005% sulfur, 3.34% silicon, 0.37% aluminium. The carbon test performed on the specimen showed a carbon content of 0.012%. To preclude nitration, only atmospheres consisting of hydrogen and water vapor were used in the experimental heat treatments. The surface was in a scale-free state produced in the cold rolling, so that the reaction took place directly between the material and the heat treatment atmosphere. The decarbonization reaction was followed by measuring the carbon monoxide content in the furnace atmosphere. The heat treatment was discontinued when no more appreciable carbon monoxide formation could be detected. The findings are shown in the figures in which,

FIG. 1 shows the effect of the water vapor content of the furnace atmosphere and of the heat treatment temperature on the carbon content that can be removed from the specimen material;

FIG. 2 shows the range of temperature and water vapor content within which the heat treatment of the invention is to be performed;

FIG. 3 shows the relationship between the carbon content that is removed by 5 minutes of heat treatment and the water vapor content of the furnace atmosphere, with the annealing temperature as a parameter; and

FIG. 4 shows a comparison of the coercive field strength in oersteds of a specimen subjected to final heat treatment by the prior art, two-step method and of the specimen treated according to the present invention.

For the heat treatment temperature of 800 C., often used in decarbonization practice, the broken line in FIG. 1 indicates the usual relationship between the carbon content of the experimental material which has been removed by the heat treatment, and the moisture of the hydrogen atmosphere. In the high moisture range the decarbonization is completely successful. If the moisture falls below about 10%, the decarbonizationability of the material diminishes rapidly, becoming virtually zero when hydrogen with a moisture content of 0.03% is used. The decarbonization of high-alloy electric sheet steel is performed technically today on the basis of this knowledge. Thus, a relatively high gas moisture is required. The formation of a thick oxide coating on the sheet surface is then inevitable.

At a heat treatment temperature of 1,100 C., decarbonization again takes place in the high moisture range, as shown by the solid curve. This range, too, is utilized in methods of the prior art which are mentioned above. If low moisture contents are used, the reaction is virtually nil over a wide range covering more than two powers of ten. Surprisingly, however, the curve climbs steeply again at very low moisture values.

In FIG. 3 the low moisture range is studied more closely. The measurements were performed at several different temperatures. At 1,050 C., in a moisture range of about 0.05%, a definite, though low maximum is already perceived. At 1,l00 C. the maximum is very pronounced and has shifted to somewhat higher moisture contents. At this temperature more than half of the initial carbon content of the specimen could be eliminated in 5 minutes. As the temperature increases further, the maximum becomes still higher and shifts further towards somewhat higher moisture contents. At a heat treatment temperature of 1,300 C. and an atmospheric moisture of about 0.4%, the carbon is removed virtually entirely.

It is noted that at very low moisture values, the decarbonization reaction vanishes again. Even if the heat treatment temperatures contain relatively little water vapor in comparison to the atmospheres used hitherto for decarbonization, and ordinarily speaking would have to be described as very dry, the water vapor content can by no means be made as small as one pleases or equal to zero. In fact, it is an important feature of the invention that the water vapor content lies within a quite specific range with definite upper and lower limits. This range is identified by single hatch lines. A boundary line marks off the crosshatched area or range in which it is possible to diminish the carbon content to less than half of the original content within a few minutes. The dots 1, 2, 3, 4, 5 mark the optimum moisture values at the corresponding temperature. They correspond to the maximums illustrated in FIG. 3. In practice, of course, the heat treatment will attempt to achieve a state that is as close as possible to the broken line joining points 1, 2, 3, 4 and 5 of FIG. 2.

The temperature range in which the decarbonization takes place coincides with the range within which there is an optimum development of the magnetic properties of the material. Consequently, the high annealing which follows the decarbonization treatment in the prior art processes is unnecessary.

The naked eye can see that much fewer reaction products are on the surface of the sheet that has been heat treated by the process of this invention than would have been expected on finished products according to the current state of the art. The better surface apparently also has a favorable effect on the soft-magnetic properties of the finished products.

FIG. 4 shows curves representing the relationship between the coercive field strength in oersteds and the annealing time in minutes in the high-annealing process used in the prior art method (broken line) and in the method of the invention (solid line) for the experimental material. Apparently, the less scale producing heat treatment results in lower magnetic hardness in the electric sheet.

The new method is primarily suited for the manufacture of non-oriented cold-rolled sheets of the initially stated composition and makes possible the economical manufacture of an electric strip or sheet steel that is improved as regards surface quality and magnetic characteristics. Even in the decarbonization of oriented sheet steel, it can be applied in regard to the heat treatment that is required in this case, especially in the case of stringent surface quality requirements.

What is claimed is:

1. A process for the simultaneous decarbonization and development of magnetic properties of cold-rolled nonoriented electrical strip or sheet steel having a silicon content between 2 and 6% by weight, which comprises continuously annealing a strip or sheet of said steel in an atmosphere contacting the entire surface of the steel and comprising water vapor and hydrogen, wherein the am nealing is at a temperature between 1,100 C. and 1,300 C. and the ratio of the partial pressure of water vapor to the partial pressure of hydrogen is in the range of 5 x10- to 2 10- at 1,100 C. and in the range of 5Xl0 to 2 1(lat 1,300 C. an in correspondingly interpolated ratios at temperatures between 1,100 C. and 1,300 C.

2. A process according to claim 1, wherein the continuous annealing is performed at a temperature and a ratio of the partial pressure of water vapor to the partial pressure of hydrogen along the line joining the dots 3, 4 and 5 of FIG. 2, the coordinates of said dots being indicated in the table above FIG. 2.

References Cited UNITED STATES PATENTS 3,640,780 2/1972 Stanley 1481 13 3,544,396 12/1970 Taylor 148113 3,127,289 3/1964 Beall 148-16 3,513,039 5/1970 Ganz 148-113 3,227,587 1/1966 Martin 1481 13 2,307,391 1/1943 Cole et al 148--1 13 2,631,118 3/1953 Boothby et al. 148122 2,287,467 6/ 1942 Carpenter et a1 148113 3,116,179 12/1963 Carpenter et a1 148-113 OTHER REFERENCES Sachs, G.: Practical Metallurgy, Cleveland, Ohio, 1951, pp. 442-443.

WALTER R. SATIERFIELD, Primary Examiner US. Cl. X.R. 

